But then WWI happened.
You mean the war that the US had no business getting involved.
Then WWII happened.
Yes, the war the American people wanted no part, but were forced into it by Roosevelt provoking Japan to attack Pearl Harbor.
It doesn't make sense to rely on Militia with infantry weapons (pistols, rifles and shotguns) to protect against tanks, planes with large bombs, Naval vessels carrying aircraft, etc.
First of all, you are basing your premise on 20th century warfare. In the modern era of nuclear weapons, the days of conventional military invasion is a thing of the past, especially that of the US, which could utterly destroy any nation that foolishly thinks it can breach America's borders. The only thing that can breach America's borders in the 21st century are the drug cartels and illegal immigrants, which the greatest military on Earth seems impotent to stop.
Second, the only reason for the US to have the standing military that it has is to garrison the planet to maintain its imperial order, which the Founding Fathers would be totally against, if they were alive today.
Third, from a cost perspective, America can no longer financially maintain its over bloated military machine. If the current pandemic has taught the militaries of the World anything is that a short-term tactical biological weaponry is the way to go to neutralize America's imperial garrisons and hamper its blue-water armadas on the planet. Hence, the US military, outside of its own nuclear and conventional ballistic missiles, is not cost-effective and actually a liability should those foreign bases are cut off due to some weaponized pathogen.
In the end, those in America who call for the preservation of the US Constitution, Bill of Rights, and the end of the imperial military are today's ideological prodigy of America's Founding Fathers, who relentlessly strive for the end of the US empire and the return of the American Republic.